PRACTICAL ORGANIC CHEMISTRY determination may be made by introducing fresh pellets of naphthalene. When the observations are complete, the apparatus is allowed to cool and the weight of benzene ascertained by weighing the boiling-tube and benzene. As in the freezing-point method, the molecular weight is calculated from the weight of substance required to raise the boiling-point of ioo grams of solvent i°, and the result multiplied by a coefficient which depends upon the nature of the solvent. The following is a list of solvents commonly employed and their coefficients and boiling-points :—'• Ether Acetone ... Chloroform Methyl alcohol Ethyl acetate b.p. k. 35° 2I'I 56° I7T 61° 36-6 66° s-s 77° 26-8 Ethyl alcohol Benzene ... Water Acetic acid Aniline bp. 78° 79° 100° 118° 184° 26-1 5*2 25'3 32-2 The molecular weight is determined from the formula M - I0° kw dW in which w is the weight of substance, W that of the solvent, d the rise of boiling-point, and k the coefficient Example.—Using the same solvent and adding successively four pellets of naphthalene, the following results were obtained :— w o o* O' o- W. d M. 66 21-313 0-l8S 126-6 93 »j 0-I85 128 3 60 3) 0-I85 126*0 01 55 0-180 132-4 Calculated for C]0H8 ; M = 128. Mean. 128-3 A simpler and more convenient form of Beckmann apparatus, requiring much less solvent and giving equally accurate results, is shown in Fig. 33. It consists of a boiling-tube furnished with two side pieces, one of which is stoppered and serves to introduce the substance and the other acts as a condenser. The boiling-tube stands on an asbestos pad and is surrounded by two short concentric glass cylinders surmounted by a mica plate. The other parts of the apparatus are similar to those in the older form and the process is conducted in the same way.